Saccharomyces cerevisiae Mod5p-II contains sequences antagonistic for nuclear and cytosolic locations

MOD5 encodes a tRNA modification activity located in three subcellular comp artments. Alternative translation initiation generates Mod5p-I, located in the mitochondria and the cytosol, and Mod5p-II, located in the cytosol and nucleus. Here we study the nucleus/cytosol distribution of overexpressed Mo d5p-II. Nuclear Mod5p-II appears concentrated in the nucleolus, perhaps ind icating that the nuclear pool may have a different biological role than the cytoplasmic and mitochondrial pools. Mod5p contains three motifs resemblin g bipartite-like nuclear localization sequences (NLSs), but only one is suf ficient to locate a passenger protein to the nucleus. Mutations of basic re sidues of this motif cumulatively contribute to a cytosolic location for th e fusion proteins. These alterations also cause decreased nuclear pools of endogenous Mod5p-II. Depletion of nuclear Mod5p-II does not affect tRNA(Tyr ) function. Despite the NLS, most Mod5p is cytosolic. We assessed whether M od5p sequences cause a karyophilic reporter to be located in the cytosol. B y this assay, Mod5p may contain more than one region that functions as cyto plasmic retention and/or nuclear export sequences. Thus, distribution of Mo d5p results from the presence/absence of mitochondrial targeting informatio n and sequences antagonistic for nuclear and cytosolic locations. Mod5p is highly conserved; sequences responsible for subcellular distribution appear to reside in "accessory" motifs missing from prokaryotic counterparts.